Khat deprecate annotate in favor of threshold

[aloctavodia]

Replace annotate argument with threshold, this makes plot_khat more consistent with plot_elpd and also allows user to define arbitrary values to use as thresholds. This also add a few tweaks to make it easier to use plot_khat with subplots and share the Y-axis.

• Follows official PR format
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1478 (32d8768) into master (8fd104c) will decrease coverage by 0.00%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master    #1478      +/-   ##
==========================================
- Coverage   91.98%   91.97%   -0.01%     
==========================================
  Files         105      105              
  Lines       11222    11239      +17     
==========================================
+ Hits        10322    10337      +15     
- Misses        900      902       +2     

Impacted Files	Coverage Δ
arviz/plots/khatplot.py	94.87% <60.00%> (-5.13%)	⬇️
arviz/plots/backends/bokeh/khatplot.py	98.59% <100.00%> (+0.08%)	⬆️
arviz/plots/backends/matplotlib/khatplot.py	75.00% <100.00%> (ø)
arviz/plots/backends/matplotlib/traceplot.py	98.80% <0.00%> (+<0.01%)	⬆️
arviz/plots/backends/matplotlib/loopitplot.py	98.76% <0.00%> (+0.01%)	⬆️
arviz/plots/backends/bokeh/traceplot.py	94.47% <0.00%> (+0.03%)	⬆️
arviz/plots/backends/bokeh/loopitplot.py	94.87% <0.00%> (+0.35%)	⬆️

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[canyon289]

LGTM code wise. Do we need to update any tests? Its fine if theyre not updated on this PR, we can open up an issue ticket to track